Kevin Systrom launched popular photo-sharing app Instagram in October—and already it boasts around 5 million users. By comparison, it took years for startups such as Facebook and Twitter to reach that growth. And Instagram, which lets users apply one-click professional filters to photographs, did so in roughly just seven months.

What most might not know is that Systrom's pre-Instagram achievments foreshadowed his eventual success. He crossed paths with Mark Zuckerberg and Adam D'Angelo when they were getting Facebook going; he interned at Twitter before Twitter was Twitter. But he left it all behind to start Instagram.

Systrom's story begins at Stanford in the early aughts, when he began developing a tool called Photobox. "I saw this problem: In college, tons of people took photos and they'd send out these huge Zip files over the Stanford email network," Systrom tells Fast Company. "That didn't really make sense: What we should do is have one place where everyone puts their photos, and can come together and basically download sets of photos if they want."

The idea caught the attention of a few young entrepreneurs who had recently arrived in the Valley. "This was when I first met Adam D'Angelo and Mark [Zuckerberg] from Facebook. When they first came out to Palo Alto, I was in this fraternity at Stanford called Sigma Nu, and through a bunch of connections, we ended up meeting those guys, time and time again," Systrom recalls. "So when I met Adam and Mark, they were like, 'Yeah, we're working on some photo stuff too, why don't you come talk to us about Facebook?'"

Keep in mind this was in 2004, before Facebook had launched its photo-sharing application, which would eventually become the world's most popular—some 200 million photos are uploaded per day, with almost 90 billion on the site in total. Yet Systrom turned down any offer. "Unfortunately, I decided I wanted to stay in school, and that's one of those decisions that I look back at—I would've loved to have been part of Facebook's growth over the years, but it was the first time I met those guys," he says. "It was certainly the harbinger for what was to come in my future."

Next, Systrom went to a little company called Odeo, "which obviously became Twitter, in college as an intern for three months," Systrom says. Like Twitter's founders, Systrom has a first-name handle (@Kevin, similar to @Jack [Dorsey] and @Biz [Stone]), and his tweets can be seen along with some of the earliest entrepreneurs involved in the company, including "Twitter's Winklevoss," Noah Glass. "Getting t [sic] ready for lunch with Systrom," Glass tweeted in June of 2006. "I loved working at a startup—I loved working with a small group of people," Systrom says of his experience.

(Regarding Glass's departure, by the way, Systrom says he wasn't around for any of it and hasn't kept in touch with Glass—although he does remember getting an email from Evan Williams that said something along the lines of, "Noah doesn't work here anymore," with no explanation.)

Ironically, after loving his startup experiences in college, Systrom didn't catch the entrepreneurship bug until he worked at an established company. After graduating, he took a job at Google, where he worked for several years, eventually finding his way into corporate development. There, he began working with startups again, and got the itch to leave and work on his own.

Of his early brushes with Facebook and Twitter, Systrom is content with how it all turned out.

"Everyone has their Facebook story, so I won't say that's necessarily unique," he says. "What I mean is that everyone has their story about how they had the chance to work at X, Y, or Z. But being at Stanford, I was given the opportunity to be in the middle of a ton of innovation, and meet some of the smartest people doing the coolest stuff in the world."